The present invention relates to a semiconductor pressure transducer incorporating a semiconductor diaphragm and, more particularly, to a semiconductor pressure transducer of the type mentioned above having an improved strength of bonding between a glass stem of a low thermal expansion coefficient in support of the semiconductor diaphragm and a holder of the glass stem.
Semiconductor pressure transducers employing a semiconductor diaphragm are disclosed, for example, in the specification of the U.S. Pat. No. 4,019,388. In the pressure transducer in this U.S. Patent specification, a silicon diaphragm in which a piezoresistance is formed is bonded by eutectic reaction by means of a solder material to a tubular holder or stem made of a borosilicate glass (7740 Pyrex). This glass holder or stem in turn is soldered to 39-42% Ni-Fe holder.
Recently, it has been proposed and actually carried out to effect the bonding between the silicon diaphragm and the glass stem and between the glass stem and the holder by anodic bonding. Such an anodic bonding is embodied, for example, in "PRESSURE TRANSDUCER" disclosed in the specification of U.S. patent application Ser. No. 81,372 filed on Oct. 3, 1979 now U.S. Pat. No. 4,321,578, assigned to the same asignee as the present application.
Also, there is a disclosure concerning the anodic bonding (or electrostatic bonding) in transducer, in an article entitled "A SOLID STATE BONDING AND PACKING TECHNIQUE FOR INTEGRATED SENSOR TRANSDUCER" (ISA ASI 73246 (229-238) 1973). According to this bonding technique, it is possible to bond two members without using any adhesive or bonding agent. In this case, however, it is essential that the materials of two members have similar thermal expansion coefficient. Usually, a borosilicate glass (for example, Pyrex glass 7740, made by Corning Glass Works) having a thermal expansion coefficient approximating that of Si is used as the material of the glass stem, while an alloy of Fe-40%Ni system is used as the material of the holder. In the pressure transducer having this construction, it is necessary that all joint parts are bonded in a gas-tight manner and at a high bonding strength, without leaving substantial residual strain after the bonding.
At temperatures below about 300.degree. C., the Pyrex glass and the Fe-40%Ni alloy exhibit approximately same thermal expansion ratio. More specifically, the Pyrex glass and the Fe-40% Ni alloy exhibit mean thermal expansion coefficient ranging between 32.times.10.sup.-7 /.degree.C. and 34.times.10.sup.-7 /.degree.C. at a temperature range between 30.degree. and 300.degree. C. As the temperature at the juncture comes down below 270.degree. C., the performance or characteristic of the glass as an electrolyte is suppressed to reduce the bonding strength. Therefore, the anodic bonding is usually conducted at a temperature between 280.degree. C. and 300.degree. C. Thus, the use of the Ni--Fe alloy restricts the range of bonding temperature. Because of a slight change in the thermal expansion coefficient of the holder material, if the bonding is made at such a temperature that the holder material exhibits a larger thermal expansion ratio than the Pyrex glass, a large stress remains in the Pyrex glass after the bonding often resulting in cracking in the Pyrex glass. Thus the above-mentioned combination of materials permits a leak of gas at the juncture and disadvantageously decreases the bonding strength. Therefore, such a combination cannot be successfully used in the semiconductor pressure transducer which requires a high precision of bonding.